Pluto's answer is correct. If you fish around with a boat hook a few feet in front of the roller, you may find a chain, attached to the drain plug.
Don't pull it 😄. Many of these drains have been sealed off in recent times to prevent a recurrence of the "Chesterfield" incident where a BW dredger caught the chain and pulled out the plug.....
There's a Martin Zero (!) video about one of these drains on the Rochdale Nine in Manchester, shows the details.
A similar roller is on the Ashton Canal near Guide Bridge, it can only be seen from a boat as there is a large removable stone over it , so can't be seen from the towpath, this drained into the Jeremy Brook but I have never seen it used and will be under several feet of silt by now.